Latitudinal differences in the amplitude of the OAE-2 carbon isotopic excursion: pCO₂ and paleo productivity

Abstract. A complete, well-preserved record of the Cenomanian/Turonian (C/T) Oceanic Anoxic Event 2 (OAE-2) was recovered from Demerara Rise in the southern North Atlantic Ocean (ODP site 1260). Across this interval, we determined changes in the stable carbon isotopic composition of sulfur-bound phytane (δ¹³C_phytane), a biomarker for photosynthetic algae. The δ¹³C_phytane record shows a positive excursion at the onset of the OAE-2 interval, with an unusually large amplitude (~7‰) compared to existing C/T proto-North Atlantic δ¹³C_phytane records (3–6‰). Overall, the amplitude of the excursion of δ¹³C_phytane decreases with latitude. Using reconstructed sea surface temperature (SST) gradients for the proto-North Atlantic, we investigated environmental factors influencing the latitudinal δ¹³C_phytane gradient. The observed gradient is best explained by high productivity at DSDP Site 367 and Tarfaya basin before OAE-2, which changed in overall high productivity throughout the proto-North Atlantic during OAE-2. During OAE-2, productivity at site 1260 and 603B was thus more comparable to the mid-latitude sites. Using these constraints as well as the SST and δ¹³C_phytane-records from Site 1260, we subsequently reconstructed pCO₂ levels across the OAE-2 interval. Accordingly, pCO₂ decreased from ca. 1750 to 900 ppm during OAE-2, consistent with enhanced organic matter burial resulting in lowering pCO₂. Whereas the onset of OAE-2 coincided with increased pCO₂, in line with a volcanic trigger for this event, the observed cooling within OAE-2 probably resulted from CO₂ sequestration in black shales outcompeting CO₂ input into the atmosphere. Together these results show that the ice-free Cretaceous world was sensitive to changes in pCO₂ related to perturbations of the global carbon cycle.